An organic thin film transistor (OTFT) has been used in electronic papers, sensors, memories, flexible displays and integrated circuits because of having a light weight, flexibility, and a low manufacturing cost. An organic semiconductor layer can be formed on a surface of an insulating substrate by spin coating and etching. Conventionally, the organic semiconductor layer may be formed by a vapor deposition method. However, in order to form a uniform organic semiconductor layer, it is necessary to form a uniform gaseous evaporating material around a depositing substrate. Since it is difficult to control a diffusion direction of atoms of the gaseous evaporating material, most of the evaporating material can not be attached to a surface of the depositing substrate, and a deposition rate of the evaporating material is slow.